Electroluminescent device with improved blue color purity

ABSTRACT

Disclosed is an electroluminescent (EL) device having a CaGa 2  S 4  :Ce luminescent layer. The ratio of the X-ray diffraction peak intensity I 2  for the (200) reflection of CaS to the X-ray diffraction peak intensity I 1  for the (400) reflection of CaGa 2  S 4  as appearing in the X-ray diffraction spectrum for the luminescent layer, I 2  /I 1 , is 0.1 or less. The amount of the impurity CaS in the luminescent layer is reduced. The EL device produces blue emission with high purity.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Applications No. 7-95215 filed on Apr. 20, 1995and No. 7-103846 filed on Apr. 27, 1995, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electroluminescent (hereinafterreferred to as EL) devices which are used in various instruments of, forexample, emissive-type segment displays and matrix displays, or indisplays in various information terminal appliances. The presentinvention also relates to methods for producing the same.

2. Related Arts

Conventional EL devices are formed by laminating a first electrode madeof an optically-transparent ITO (indium tin oxide) film, a firstinsulating layer comprising Ta₂ O₅ (tantalum pentoxide) or the like, aluminescent layer, a second insulating layer, and a second electrodemade of an ITO film, on an insulating glass substrate in that order.

The luminescent layer comprises, for example, a host material such asZnS (zinc sulfide) with a luminescent center such as Mn (manganese) orTb (terbium) added thereto, or a host material such as SrS (strontiumsulfide) with a luminescent center such as Ce (cerium) added thereto.

EL devices give different colors depending on the choice of theadditives in ZnS. For example, those having Mn as the luminescent centerproduce yellowish orange colors, while those having Tb produce greencolors. EL devices having Ce as the luminescent center in SrS producebluish green colors.

To realize full-color EL displays, luminescent layers capable ofproducing red, green and blue colors must be formed. Of these, SrS witha luminescent center of Ce is generally used as the material for theblue-emitting layers in EL devices. However, this material naturallyproduces a bluish green color. Therefore, in order to attain pure blueemission, a filter capable of cutting off the green component out of theemission spectrum must be used.

As opposed to this, it is known that an EL device with a CaGa₂ S₄ :Celuminescent layer having Ce as the luminescent center element added tothe host CaGa₂ S₄ (calcium thiogallate) can produce a blue color withoutusing a filter, for example, as so reported in SID 93 Digest, pp.761-764 (1993).

However, it is reported in the reference that the CIE (CommissionInternationale de l'Eclairage) chromaticity coordinates for the ELdevice with the CaGa₂ S₄ :Ce luminescent layer indicate x=0.15 andy=0.19. On the other hand, the CIE chromaticity coordinates for ZnS:Agthat is used for the blue phosphor in cathode-ray tubes indicate X=0.15and y=0.07 or so. The purity of the blue color produced by theconventional CaGa₂ S₄ :Ce luminescent layer is low and therefore theluminescent layer was unsatisfactory.

SUMMARY OF THE INVENTION

Considering the above-mentioned problems, the present inventors haveaccomplished the present invention, and an object of the presentinvention is to improve the blue color purity in EL devices withthiogallate luminescent layer containing calcium, such as CaGa₂ S₄ :Celuminescent layer.

The present inventors have found from their extensive researches andrepetitive experiments that the X-ray diffraction spectrum for the CaGa₂S₄ :Ce luminescent layer as produced by the conventional method shows adiffraction peak for CaS. The existence of CaS results in thegreen-emitting component due to CaS:Ce, which lowers the blue colorpurity. Therefore, the decrease, if possible, in the green-emittingcomponent resulting from CaS:Ce will contribute to the increase in theblue color purity. On the basis of these discoveries, the presentinventors have completed the present invention.

According to the present invention, a CaGa₂ S₄ :Ce luminescent layer isselected to have a film quality characterized in that the ratio of theX-ray diffraction peak intensity I₂ for the (200) reflection of CaS tothe X-ray diffraction peak intensity I₁ for the (400) reflection ofCaGa₂ S₄ as appearing in the X-ray diffraction spectrum for theluminescent layer, I₂ /I₁, is 0.1 or less (including 0).

Such CaGa₂ S₄ :Ce luminescent layers can be deposited by means of asputtering method using a sintered target which comprises as a maincomponent CaGa₂ S₄ doped with Ce and presents a ratio of the X-raydiffraction peak intensity I₂ for the (200) reflection of CaS to theX-ray diffraction peak intensity I₁ for the (400) reflection of CaGa₂ S₄as appearing in the X-ray diffraction spectrum, I₂ /I₁, that is 0.5 orless (including 0).

Other objects and features of the present invention will appear in thecourse of the description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and characteristics of the presentinvention will be appreciated from a study of the following detaileddescription, the appended claims, and drawings, all of which form a partof this application. In the drawings:

FIG. 1 is a schematic view showing the cross-section of an embodiment ofthe EL device of the present invention;

FIG. 2 is a graph showing the X-ray diffraction spectrum for a sourcematerial powder CaGa₂ S₄ which is used for the production of a sinteredtarget;

FIG. 3 is a graph showing the X-ray diffraction spectrum for theluminescent layer formed with the sintered target formed from the sourcematerial powder that gave the spectrum of FIG. 2;

FIG. 4 is a graph showing the variation in the value y for the CIEchromaticity coordinates relative to the variation in the ratio I₂ /I₁for the formed luminescent layers;

FIG. 5 is a graph showing the variation in the value y for the CIEchromaticity coordinates relative to the variation in the ratio I₂ /I₁for the used sintered targets;

FIG. 6 is a schematic view showing the cross-section of the sinteredtarget used for forming the luminescent layer;

FIG. 7 is a graph showing the relationship between the Ce concentrationin the formed luminescent layer and the relative density of the usedsintered target;

FIG. 8 is a graph showing the relationship between the amount of Ga₂ S₃added to the sintered target and the ratio of Ga/Ca in the formedluminescent layer;

FIG. 9 is a graph showing the X-ray diffraction spectrum for theluminescent layer according to a fourth embodiment of the presentinvention; and

FIG. 10 is a graph showing the variation in the value y for the CIEchromaticity coordinates relative to the variation in the ratio I₂ /I₃for the formed luminescent layers of the fourth embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

The EL device according to the present invention is characterized, asdescribed above, by comprising a CaGa₂ S₄ :Ce luminescent layer whichassumes in an X-ray diffraction spectrum thereof a diffraction peak forCaS controlled to improve the blue color purity.

More specifically, the first aspect of the invention resides in an ELdevice comprising a first electrode, a first insulating layer, aluminescent layer, a second insulating layer and a second electrode, alllaminated in that order on a substrate in such a manner that the side ofthe device through which the light from the luminescent layer is emittedis optically transparent, the device being characterized in that theluminescent layer comprises CaGa₂ S₄ doped with Ce as the luminescentcenter and that the ratio of the X-ray diffraction peak intensity I₂ forthe (200) reflection of CaS to the X-ray diffraction peak intensity I₁for the (400) reflection of CaGa₂ S₄ as appearing in the X-raydiffraction spectrum for the luminescent layer, I₂ /I₁ is 0.1 or less(including 0).

The definition of the ratio of the CaS diffraction peak intensity to theCaGa₂ S₄ diffraction peak intensity as appearing in the X-raydiffraction spectrum for the luminescent layer to the predeterminedvalue or less restricts the amount of the impurity CaS existing in theluminescent layer to a predetermined value or less. As a result, thegreen-emitting component to be caused by CaS:Ce is decreased, therebyproducing a blue color with high purity.

The second aspect of the invention resides in a method for producing theEL device, which is characterized in that a sintered target comprisingCaGa₂ S₄ doped with Ce as a luminescent center element to assume a ratioof the X-ray diffraction peak intensity I₂ for the (200) reflection ofCaS to the X-ray diffraction peak intensity I₁ for the (400) reflectionof CaGa₂ S₄ as appearing in the X-ray diffraction spectrum, I₂ /I₁, ofbeing 0.5 or less (including 0) is used for forming the luminescentlayer by sputtering.

The use of the sputtering target having such a reduced content of theimpurity CaS contributes to the decrease in the proportion of CaS thatwill grow in the luminescent layer, resulting in the formation of theluminescent layer having a reduced amount of CaS therein. Therefore, theEL device obtained by this method produces a blue color with highpurity.

As the third aspect of the invention, the sintered target is produced byintroducing a gallium compound into the essential component of CaGa₂ S₄.The introduction of a gallium compound makes the amount of Ga in theluminescent layer appropriate, whereby the luminescent layer produces ablue color with higher purity. The gallium compound includes GaS, Ga₂ S₃and Ga₂ O₃, one or more of which may be used.

For Ga₂ S₃ or Ga₂ O₃, it is desirable that its amount to be added fallsbetween 2 mol % and 12 mol % relative to CaGa₂ S₄. If so, thestoichiometric ratio of Ga to Ca may be about 2, resulting in thesuccessful growth of CaGa₂ S₄ to realize the intended pure blueemission.

As the fourth aspect of the invention, the ratio of the X-raydiffraction peak intensity I₂ for the (200) reflection of CaS to theX-ray diffraction peak intensity I₁ for the (400) reflection of CaGa₂ S₄as appearing in the X-ray diffraction spectrum for the source materialpowder of CaGa₂ S₄ that is used for the production of the sinteredtarget, I₂ /I₁, is 0.5 or less (including 0). The increase in the purityof the source material powder of CaGa₂ S₄ to be used results in anincrease in the purity of the sintered target, with which is formed theluminescent layer that produces a blue color with higher purity.

As the fifth aspect of the invention, the density of the sintered targetto be used for the sputtering is defined to be at least 75% of thedensity of the single crystal of the essential material CaGa₂ S₄. Theintroduction of the gallium compound into the sintered target as in thethird aspect of the invention often causes variation in the density ofthe sintered target that depends on the amount of the gallium compoundadded. In this connection, the present inventors have discovered acorrelation between the density of the sintered target and the Ceconcentration to be in the luminescent layer which is such that thelower the density the smaller the Ce concentration. In order to evadethis disadvantage, it is preferable that the density of the sinteredtarget should be 75% or more of the density of the single crystal ofCaGa₂ S₄ to thereby prevent the variation in the Ce concentration to bein the luminescent layer and stabilize the high Ce concentration to betherein. This results in the stabilization of the emission luminance.

As the sixth aspect of the invention, the gallium compound is added tothe essential material CaGa₂ S₄ as its oxide, and the luminescent layeris formed in a sputtering gas containing a reducing gas. The addition ofsuch a gallium oxide makes it easy to produce a sintered target having ahigh relative density. This is because oxides are extremely stable towater unlike sulfides that are easily hydrolyzed.

The addition of a gallium oxide is inevitably accompanied by theintroduction of oxygen into the sintered target. However, theintroduction of oxygen into the luminescent layer can be prevented byusing a sputtering gas containing a reducing gas, and the lowering ofthe emission luminance is thereby prevented. As the reducing gas,employable is hydrogen sulfide. The hydrogen sulfide content of thesputtering gas may be at least 5 mol %, with which the substantialintroduction of oxygen into the luminescent layer is surely prevented.More desirably, the hydrogen sulfide content is from 5 mol % to 30 mol%.

As the seventh aspect of the invention, the edges of the sintered targetare chamfered or round-cornered. The sintered target with an excessamount of a gallium compound as above is more difficult to mold thanthat with no gallium compound and is often more brittle than the latter.The brittleness causes cracks of the edges of the sintered target,resulting in abnormal discharging during sputtering and in unevenfilming of the luminescent layer. However, the processing of the edgesof the sintered target as above prevents the edges of the target frombeing cracked and also prevents the abnormal discharging. It isdesirable that the chamfering is conducted to a degree of from C 0.01 mmto C 2.0 mm or the round-cornering is from R 0.01 mm to R 2.0 mm.

The eighth aspect of the invention resides in a method for producing theEL device, which is characterized in that a source material of aII-IIIb-VIb compound is mixed with single substances or compounds of aluminescent center element and of the group IIIb element constitutingthe source material and sintered to give a sintered target having adensity of 75% or more of the density of the single crystal of thesource material, and the sintered target is sputtered to form theluminescent layer.

Using the sintered target thus having a relative density of 75% or moreof the density of the single crystal of the II-IIIb-VIb compound, thevariation in the concentration of the luminescent center element in theluminescent layer is prevented while the concentration is stabilizedhigh.

As the group II element, employable are one or more selected from Ca,Sr, Ba and Zn. One or more selected from Al, Ga and In are employable asthe group IIIb element. One or more selected from S and Se areemployable as the group VIb element. When the group IIIb element is Gaand the group VIb element is S, the host for the luminescent layer maycomprise alkaline earth thiogallates (MGa₂ S₄ in which M=Ca, Sr, Ba).

The luminescent center element may be either a transition metal elementor a rare earth element. Mn may be employed as the transition metalelement, and Ce or Eu (europium) as the rare earth element.

The compounds of the group IIIb element to be added includes oxides,sulfides, selenides and their mixtures. The sulfide may be Ga₂ S₃ or GaS(gallium(II) sulfide); and the oxide may be Ga₂ O₃ (gallium(III) oxide).

The compounds of the group IIIb element also includes Al₂ S₃ (aluminumsulfide), Al₂ O₃ (aluminium(III) oxide) and their mixtures.

As one preferred embodiment of the invention, Ga₂ S₃ or Ga₂ O₃ is addedto a II-IIIb-VIb compound, the chemical formula of which is representedas II-IIIb₂ -VIb₄, in an amount of from 2 mol % to 12 mol %.

Using the II-IIIb₂ -VIb₄ compound with from 2 to 12 mol % of Ga₂ S₃ orGa₂ O₃, the stoichiometric ratio of the group IIIb element to the groupII element in the luminescent layer formed may be about 2, resulting inthe successful growth of II-IIIb₂ -VIb₄ in the layer with which the ELdevice produces the intended color emission.

Hereafter, preferred embodiments of the present invention will beexplained with reference to the accompanying drawings.

FIG. 1 is a schematic view showing the cross-section of a thin film ELdevice 10 as one embodiment of the invention, from which the emittedlight is emitted in the arrowed directions.

The thin film EL device 10 comprises a first transparent electrode(first electrode) 2 made of an optically-transparent ZnO (zinc oxide), afirst insulating layer 3 made of an optically-transparent SrTiO₃(strontium titanate), a luminescent layer 4 made of CaGa₂ S₄ with aluminescent center Ce, a second insulating layer 5 made of anoptically-transparent SrTiO₃, and a second transparent electrode (secondelectrode) 6 made of an optically-transparent ZnO, all laminated in thatorder on an insulating glass substrate 1.

The thickness of the first and second transparent electrodes 2 and 6 is300 nm each, that of the first and second insulating layers 3 and 5 is500 nm each, and that of the luminescent layer 4 is 600 nm. Thethickness of each layer is based on the site in the center of the glasssubstrate 1.

Next, the first embodiment of the production of the thin film EL device10 is mentioned below.

First, a film for the first transparent electrode 2 is formed on theglass substrate 1. A pellet as formed by mixing a ZnO powder with Ga₂ O₃(gallium oxide) followed by shaping the resulting mixture is used forthe vaporizing material. An ion-plating device is used for the formationof the film. Concretely, the ion-plating device is evacuated while theglass substrate 1 therein is kept at a pre-determined temperature. Afterthis, argon (Ar) gas is introduced into the device to keep the pressuretherein at a pre-determined value, and the beam power and the RF powerare adjusted to make the filming rate fall between 6 and 18 nm/min., forexample.

Next, the first insulating layer 3 of SrTiO₃ is formed on the firsttransparent electrode 2 by sputtering. Concretely, a mixed gascomprising Ar and O₂ (oxygen) is introduced into the sputtering devicewhile the glass substrate 1 is kept at a pre-determined temperature, anda film of the layer 3 is formed at an RF power of 1 kW.

The CaGa₂ S₄ :Ce (calcium thiogallate doped with cerium) luminescentlayer 4 comprising a host of CaGa₂ S₄ with a luminescent center of Ce isformed on the first insulating layer 3, by sputtering.

Concretely, a mixed gas comprising Ar and 20 mol %, relative to Ar, ofH₂ S (hydrogen sulfide) is introduced into the sputtering chamber whilethe glass substrate 1 therein is kept at a constant temperature of 300°C., and the luminescent layer 4 is deposited at an RF power of 300 W. Asthe sputtering target, used is a sintered target CaGa₂ S₄ :Ce with Ce asthe luminescent center. It is desirable that the H₂ S concentration inthe sputtering gas is from 5 mol % to 30 mol %.

Next, the thus-sputtered CaGa₂ S₄ :Ce luminescent layer 4 isheat-treated in an Ar atmosphere containing 20 mol % of H₂ S, at 630° C.for 30 minutes. As a result, the CaGa₂ S₄ :Ce luminescent layer 4, whichis in amorphous state and therefore is not luminescent just after theformation, becomes crystallized and luminescent.

Next, the second insulating layer 5 of SrTiO₃ is formed on theluminescent layer 4 in the same manner as in the formation of the firstinsulating layer 3. Then, the second transparent electrode 6 of a ZnOfilm is formed on the second insulating layer 5 in the same manner as inthe formation of the first transparent electrode 2.

The X-ray diffraction spectrum for the thin film EL device 10 thusproduced in the manner mentioned above was investigated. The diffractionpeak intensity in the X-ray diffraction spectrum for the (400)reflection of CaGa₂ S₄ is herein referred to as I₁, and that for the(200) reflection of CaS as I₂.

To produce the sintered target, used was a source material powder ofCaGa₂ S₄ with high purity to have a ratio of I₂ /I₁ of nearly 0, asshown by the X-ray diffraction spectrum of FIG. 2. Two wt. % of CeF₃(cerium fluoride) was added thereto to form the luminescent center, andthe powder was sintered in H₂ S and formed into a sintered target by aknown method.

The X-ray diffraction spectrum for the thus-formed sintered targetexhibited the same characteristics as those in the X-ray diffractionspectrum shown by FIG. 2, in which, therefore, I₂ /I₁ was nearly 0.

FIG. 3 shows the X-ray diffraction spectrum for the luminescent layer 4as formed from the sintered target by sputtering followed by heattreatment. The ratio of I₂ /I₁ in the luminescent layer was about 0.05.The CIE chromaticity coordinates for the EL device 10 thus produced aresuch that x=0.15 and y=0.17, and the purity of the blue color from theEL device 10 was higher than that from the conventional EL device.

By lowering the ratio I₂ /I₁ in the luminescent layer in that manner,the green-emitting component to be caused by CaS:Ce is decreased andblue-EL emission with high purity is attained.

In another embodiment (the second embodiment detailed hereinunder), 5wt. % of Ga₂ S₃ (gallium (III) sulfide) was added to the source materialpowder used hereinabove to produce the sintered target. The ratio I₂ /I₁in the sintered target thus produced was nearly 0.

The ratio I₂ /I₁ in the luminescent layer 4 as formed by sputtering thesintered target and followed by heat treatment was less than 0.01. TheCIE chromaticity coordinates for the EL device with the layer 4 weresuch that x=0.15 and y=0.16. Thus, the addition of Ga₂ S₃ makes theamount of Ga in the luminescent layer more appropriate, by which thepurity of the blue color from the luminescent layer was much moreincreased than the above.

Apart from Ga₂ S₃ used in the above, the excess gallium compound to beadded to the essential component CaGa₂ S₄ may be GaS (gallium(II)sulfide) or Ga₂ O₃ (gallium(III) oxide) or even a mixture of these. Itis desirable that the amount of the gallium compound to be added is from1 wt. % to 10 wt. %.

Next, the relationship between the ratio I₂ /I₁ in the luminescent layerand the purity of blue color emission is referred to below.

CaS was further intentionally added to the source material powder CaGa₂S₄ with I₂ /I₁ of nearly 0, such as that used in the first embodiment,and formed into sintered targets. Using these, luminescent layers withvarying I₂ /I₁ ratios were formed by sputtering and their color emissionwas investigated.

FIG. 4 shows the variation in the value y for the CIE chromaticitycoordinates relative to the variation in the ratio I₂ /I₁ for theluminescent layers formed. It has been confirmed that the value x forthe coordinates did not change and was constantly 0.15 irrespective ofthe variation in the ratio I₂ /I₁. As is known from FIG. 4, y=0.19 inthe conventional product became y=0.16 or so with the decrease in theratio I₂ /I₁ in the luminescent layers. Therefore, the purity of blueemission from the luminescent layers with I₂ /I₁ of 0.1 or less shall behigher than that from the conventional luminescent layers.

Next, the ratio I₂ /I₁ in the sintered target as well as the sourcematerial powder CaGa₂ S₄ which is used for the production of thesintered target are investigated hereinunder.

Like the above, CeF₃ was added to the source material powder CaGa₂ S₄with I₂ /I₁ of nearly 0 and, in addition, CaS was intentionally addedthereto, thereby varying the ratio I₂ /I₁ in the resulting sourcematerial powders. The ratio I₂ /I₁ in the source material powders wasalmost the same as that in the sintered targets therefrom. Using thesintered targets thus having varying I₂ /I₁ ratios, EL devices wereproduced and their EL emission was investigated. In this investigation,used was a source material powder CaGa₂ S₄ not containing Ga₂ S₃.

FIG. 5 shows the variation in the value y for the CIE chromaticitycoordinates relative to the variation in the ratio I₂ /I₁ for thesintered targets formed. As is known from FIG. 5, y=0.19 in theconventional product became y=0.17 or so with the decrease in the ratioI₂ /I₁ in the sintered targets. Therefore, the purity of blue emissionfrom the luminescent layers made from the sintered targets with I₂ /I₁of 0.5 or less shall be higher than that from the conventionalluminescent layers. The addition of Ga₂ S₃ to the source material powderCaGa₂ S₄ resulted in the higher improvement in the purity of blueemission than that in the results shown in FIG. 5.

The above-mentioned embodiment demonstrated the production of CaGa₂ S₄:Ce EL devices. Also for other SrGa₂ S₄ :Ce EL devices and BaGa₂ S₄ :CeEL devices, the purity of blue emission was improved by restricting thecontent of SrS and BaS to a pre-determined value or lower.

Next, a second embodiment of the invention is mentioned below, in which,as an additive Ga compound, Ga₂ S₃ is mixed with a source materialpowder of CaGa₂ S₄ and formed into a sintered target and the target isused to form the luminescent layer 4.

Like in the above-mentioned first embodiment, the first transparentelectrode 2 of ZnO and the first insulating layer 3 of SrTiO₃ are formedin that order on the glass substrate 1 (FIG. 1 is referred to).

Next, the CaGa₂ S₄ :Ce luminescent layer 4 is formed on the firstinsulating layer 3 by sputtering. Concretely, a mixed gas comprising Arand 20 mol %, relative to Ar, of H₂ S (hydrogen sulfide) is introducedinto the deposition chamber while the glass substrate 1 therein is keptat a constant temperature of 300° C., and the film deposition isconducted at an RF power of 300 W.

The sputtering target used herein is prepared by adding 4 mol % of CeF₃and 6 mol % of Ga₂ S₃ to a source material CaGa₂ S₄ followed bysintering the resulting mixture. The sintered target is formed to have arelative density of 80%. The relative density as referred to herein isobtained relative to the density of the single crystal of the essentialmaterial, i.e., CaGa₂ S₄. As the density of the single crystal of CaGa₂S₄, employed is a value of 3.38 g/cm³ as reported by T. E. Peters etal., in Journal of Electrochemical Society, Vol. 119, No. 2, pp.230-236.

The edges of the sintered target is chamfered at C 1.0 mm, as in FIG. 6which shows the cross-section of the target. The chamfering is effectivein preventing the edges from being cracked. It is desirable that thechamfering is conducted at from C 0.01 mm to C 2.0 mm. In FIG. 6, A isthe sintered target and B is a backing plate.

The chamfering may be substituted with round-cornering to attain thesame effect. The round-cornering is desirably conducted at from R 0.01mm to R 2.0 mm.

Next, the deposited layer 4 is heat-treated in an Ar atmospherecontaining 20 mol % of H₂ S, at 650° C. for 5 minutes. As a result, theCaGa₂ S₄ :Ce luminescent layer 4, which is in amorphous state andtherefore is not luminescent just after the deposition, becomescrystallized and luminescent.

After this, the second insulating layer 5 of SrTiO₃ and the secondtransparent electrode 6 of ZnO are formed in that order on theluminescent layer 4, in the same manner as in the first embodiment.

The Ce concentration in the luminescent layer 4 thus formed in themanner as above was measured with an EPMA (electron probe microanalyzer)to be 0.39±0.02 atm %.

The ratio of the X-ray diffraction peak intensity I₂ for the (200)reflection of CaS to the X-ray diffraction peak intensity I₁ for the(400) reflection of CaGa₂ S₄ as appearing in the X-ray diffractionspectrum for the luminescent layer 4 as formed by sputtering in thesecond embodiment, I₂ /I₁, was 0.05.

Next, various sintered targets were prepared, and the relationshipbetween the relative density of the target and the Ce concentration inthe resultant luminescent layer was investigated. Sintered targets withvarying relative densities were prepared by hot-pressing. The resultsfrom the investigation are shown in FIG. 7. From FIG. 7, it is knownthat the sintered targets with relative density of 75% or more presentedalmost no difference in the Ce concentration in the luminescent layersto be formed therefrom but those with relative density of less than 75%led to the noticeable decrease in the Ce concentration. Therefore, inorder to introduce Ce into the luminescent layers with highreproducibility, it is effective to make the sintered targets have arelative density of 75% or more. Furthermore, the sintered targets withrelative density of 75% or more were less brittle.

FIG. 8 shows the ratio Ga/Ca in the luminescent layers relative to theamount of Ga₂ S₃ added. To attain the successful growth of CaGa₂ S₄, theratio Ga/Ca must be near to the stoichiometric ratio of 2. Theexperiments verified the successful result within the range between 1.8and 2.4 for the ratio Ga/Ca. This is because the ratio Ga/Ca of smallerthan 1.8 results in the preferential growth of CaS but the ratio Ga/Caof larger than 2.4 results in the preferential growth of Ga₂ S₃. Thepreferential growth of CaS makes the blue color purity lowered becauseof the green-emitting component caused by CaS:Ce. On the other hand, thepreferential growth of Ga₂ S₃ markedly hinders the growth of CaGa₂ S₄.In this case, although the blue emission is surely obtained, theluminescence threshold voltage thereof inevitably increases, andfurther, the luminance-voltage characteristic curve becomes degraded ingradient. Accordingly, the obtained luminance is low under the sameapplied voltage. Therefore, from FIG. 8, it is noted that the amount ofGa₂ S₃ to be added must be from 2 mol % to 12 mol %. The same shallapply to Ga₂ O₃ that is added in place of Ga₂ S₃ in a third embodimentto be mentioned below.

Herein, because the growth of CaS in a case of the ratio Ga/Ca ofsmaller than 1.8 depends upon the purity of the sintered target, Gacompound to be added may be less than 2 mol % when the purity of thesource material powder of CaGa₂ S₄, which is the source material of thesintered target, is high.

The substitution of SrGa₂ S₄ for CaGa₂ S₄ in the second embodiment gavethe same results as above. Concretely, the sintered targets with SrGa₂S₄ having a relative density of 75% or more formed luminescent layershaving high and stable Ce concentrations, while those having a relativedensity of less than 75% formed luminescent layers having noticeablyreduced Ce concentrations. As the density of the single crystal of SrGa₂S₄, used was a value of 3.61 as reported in the above-mentionedreference.

A third embodiment of the invention is mentioned below, in which Ga₂ O₃is added in place of Ga₂ S₃ in the second embodiment to produce thesintered targets.

Almost nothing of the sintered targets produced in the third embodimentwhere Ga₂ O₃ was added had a relative density of 70% or less. This isconsidered because Ga₂ S₃ used in the second embodiment is susceptibleto hydrolysis, causing the poor reproducibility in the production of thesintered targets. As opposed to this, since Ga₂ O₃ is extremely stableto water, its addition makes it easy to produce high-density sinteredtargets.

However, the use of the sintered targets with Ga₂ O₃ is problematic inthat oxygen is introduced in the luminescent layer 4 from Ga₂ O₃. Theintroduction of oxygen into the luminescent layer 4 retards thecrystallization of the luminescent layer 4 in its heat treatment,thereby lowering the luminance of the formed EL device. In order tosolve this problem, a reducing gas such as H₂ S is introduced into thesputtering gas for the formation of the luminescent layer 4 in thisthird embodiment.

The experiments verified that the EL devices formed with Ga₂ O₃ of 6 mol% in the absence of H₂ S lost their luminescence but those formed in thepresence of 5 mol % of H₂ S were successfully luminescent. Therefore,the introduction of 5 mol % or more of H₂ S into the sputtering gas forEL devices with Ga₂ O₃ substantially prevents the introduction of oxygeninto the luminescent layers and is effective in successfully producingEL devices.

The same relationship between the relative density of the sinteredtarget and the Ce concentration in the luminescent layer as that shownin FIG. 7 was observed also for the sintered targets with Ga₂ O₃.Concretely, the sintered targets with relative density of 75% or moreformed luminescent layers with reduced variation in the Ce concentrationwith good reproducibility.

If stable Ga compounds are desired, GaS may be used in place of Ga₂ O₃.

Although the above-mentioned embodiments demonstrated the production ofCaGa₂ S₄ :Ce EL devices, the present invention is not limited to theCaGa₂ S₄ :Ce luminescent layer. Ca in the compound may be substituted,for example, with Sr to give the chemical formula Ca_(1-p) Sr_(p) Ga₂ S₄:Ce which are within the scope of the invention. Next, a fourthembodiment of the invention is mentioned below, in which Ca_(1-p) Sr_(p)Ga₂ S₄ :Ce layer is formed as a luminescent layer.

The luminescent layer 4 composed of Ca₀.5 Sr₀.5 Ga₂ S₄ :Ce will beexplained as a typical example of the fourth embodiment. As the fourthembodiment has the same structure as that of the first embodiment shownin FIG. 1 other than the material of the luminescent layer 4, theexplanation below will be made with reference to FIG. 1.

As in the foregoing embodiments, on the glass substrate 1 is formed thefirst transparent electrode 2 of a ZnO film and the first insulatinglayer 3 of SrTiO₃ in that order. Then, the luminescent layer 4 is formedon the first insulating layer 3 by sputtering.

The sputtering target used herein is prepared by: mixing CaGa₂ S₄ withthe iso-molar amount of SrGa₂ S₄ ; adding 4 mol % of CeF₃ and 4 mol % ofGa₂ S₃ to the CaGa₂ S₄ and SrGa₂ S₄ mixture; and sintering in hydrogensulfide atmosphere the resultant mixture. Here, to produce the sinteredtarget, used was a source material powder of CaGa₂ S₄ with high purity,which has the ratio of the X-ray diffraction peak intensity I₂ for the(200) reflection of CaS to the X-ray diffraction peak intensity I₁ forthe (400) reflection of CaGa₂ S₄, I₂ /I₁, of nearly 0, as in the firstembodiment (see the X-ray diffraction spectrum of FIG. 2).

The composition of the thus-formed sintered target is given by Ca₀.5Sr₀.5 Ga₂ S₄ :Ce, and the X-ray diffraction spectrum therefor alsoshowed I₂ /I₁ of nearly 0. However, the X-ray diffraction spectrum forthe sintered target exhibited characteristics having peaks correspondingto CaGa₂ S₄ and SrGa₂ S₄, respectively. Although peaks corresponding toSrGa₂ S₄ appear in the X-ray diffraction spectrum, the purity of blueemission from the luminescent layers made from the sintered targets withI₂ /I₁ of 0.5 or less will be improved like in the first embodiment.

Furthermore, the sintered target formed had a relative density of 80%.The relative density as referred to herein is defined as an averagevalue calculated in relation to the density of the single crystal ofCaGa₂ S₄ and the density of the single crystal of SrGa₂ S₄, both beingcited in the abovementioned document, and in association with the mixingratio of calcium and strontium thiogallates. In this fourth embodiment,as the same molar amount of CaGa₂ S₄ and SrGa₂ S₄ are mixed, using thedensity of the single crystal of CaGa₂ S₄ of 3.38 g/cm³, the density ofthe single crystal of SrGa₂ S₄ of 3.61 g/cm³, the molecular weight ofCaGa₂ S₄ of 307.8 and the molecular weight of SrGa₂ S₄ of 355.34, thedensity of the single crystal of Ca₀.5 Sr₀.5 Ga₂ S₄, ρ, is calculatedas: ##EQU1##

Using the calculated density of the single crystal of Ca₀.5 Sr₀.5 Ga₂S₄, ρ, the relative density of the sintered target is obtained.

Furthermore, the same relationship between the relative density of thesintered target and the Ce concentration in the luminescent layer asthat shown in FIG. 7 was observed also for the sintered targets of thecomposition Ca₀.5 Sr₀.5 Ga₂ S₄ with Ga₂ S₃. Here, even if thecompositional ratio p in Ca_(1-p) Sr_(p) Ga₂ S₄ varies, the samecharacteristic as that shown in FIG. 7 can be obtained.

The sputtering to deposit the Ca₀.5 Sr₀.5 Ga₂ S₄ :Ce luminescent layer 4is performed using above-mentioned sintered target under the conditionsequal to the first embodiment.

Concretely, a mixed gas comprising Ar and 20 mol %, relative to Ar, ofH₂ S (hydrogen sulfide) is introduced into the sputtering chamber whilethe glass substrate 1 therein is kept at a constant temperature of 300°C., and the luminescent layer 4 is deposited at an RF power of 300 W.

Next, the thus-deposited Ca₀.5 Sr₀.5 Ga₂ S₄ :Ce luminescent layer 4 isheat-treated in an Ar atmosphere containing 20 mol % of H₂ S, at 630° C.for 30 minutes. As a result, the Ca₀.5 Sr₀.5 Ga₂ S₄ :Ce luminescentlayer 4, which is in amorphous state and therefore is not luminescentjust after the deposition, becomes crystallized and luminescent.

Next, the second insulating layer 5 of SrTiO₃ is formed on theluminescent layer 4, and then, the second transparent electrode 6 of aZnO film is formed on the second insulating layer 5 in the same manneras in the first embodiment.

The X-ray diffraction spectrum for the thin film EL device 10 thusproduced in the manner mentioned above was investigated.

FIG. 9 shows the X-ray diffraction spectrum for the Ca₀.5 Sr₀.5 Ga₂ S₄:Ce luminescent layer 4 as formed from the sintered target by sputteringfollowed by heat treatment. As is understood from FIG. 9, peaks of thediffraction intensity representing SrGa₂ S₄ appear whereas peaks of thediffraction intensity representing CaGa₂ S₄ are absent. Additionally,differing from the first embodiment, the X-ray diffraction spectrumshown in FIG. 9 verifies that the diffraction peak intensity for the(422) reflection of SrGa₂ S₄ tends to be a main peak in case of theCa₀.5 Sr₀.5 Ga₂ S₄ :Ce luminescent layer.

Therefore, in case the Ca_(1-p) Sr_(p) Ga₂ S₄ :Ce luminescent layer isformed, the ratio of the X-ray diffraction peak intensity I₂ for the(200) reflection of CaS to the X-ray diffraction peak intensity I₃ forthe (422) reflection of SrGa₂ S₄, I₂ /I₃, should be evaluated.

The ratio of I₂ /I₃ in the luminescent layer was about 0.07. The CIEchromaticity coordinates for the EL device 10 thus produced were suchthat x=0.15 and y=0.14, and the purity of the blue color from the ELdevice 10 was higher than that from the EL device of the firstembodiment.

In the above explanation of the fourth embodiment, the Ca₀.5 Sr₀.5 Ga₂S₄ :Ce luminescent layer is mainly mentioned as a typical example;however, the compositional ratio p in the chemical formula Ca_(1-p)Sr_(p) Ga₂ S₄ can be changed. Here, it is preferable that thecompositional ratio p should be from 0.15 to 0.6 inclusive (0.15≦p≦0.6),which results in luminescence with a stable luminance.

Next, the relationship between the ratio I₂ /I₃ in the luminescent layerand the purity of blue color emission is referred to below.

CaS was further intentionally added to the source material of the targetinitially showing I₂ /I₁ of nearly 0, such as that used in the fourthembodiment, and formed into sintered targets. Using these, luminescentlayers with varying I₂ /I₃ ratios were formed by sputtering and theircolor emission was investigated.

FIG. 10 shows the variation in the value y for the CIE chromaticitycoordinates relative to the variation in the ratio I₂ /I₃ for theluminescent layers formed. It has been confirmed that the value x forthe coordinates did not change and was constantly 0.15 irrespective ofthe variation in the ratio I₂ /I₃. As is known from FIG. 10, the value yfor the CIE chromaticity coordinates sharply increases when the ratio I₂/I₃ exceeds 0.1, which degrades the blue color purity due to thegreen-emitting component caused by CaS:Ce. Therefore, it is desirablethat the ratio I₂ /I₃ is controlled to be 0.1 or less.

As described above, by lowering the ratio I₂ /I₃ in the luminescentlayer, or, in the alternative, by lowering the ratio I₂ /I₁ in thesintered target, the green-emitting component to be caused by CaS:Ce isdecreased and blue-EL emission with high purity is attained.

Furthermore, discussing the present invention with reference to thesputtering targets to be used, the targets may be composed of CaGa₂ S₄:Ce with a gallium compound of Ga₂ S₃ or Ga₂ O₃ but are not limited toonly these compositions.

When Eu, Mn or the like was used in place of Ce as the luminescentcenter element, 75% or more of the relative density of the sinteredtarget attained a high stabilized concentration of the luminescentcenter element in the formed luminescent layers. In case BaGa₂ S₄, SrGa₂S₄, CaAl₂ S₄, ZnIn₂ S₄ and the like were used as the source materialsfor the targets, the same results as above was also verified.

Summarizing these, therefore, the sputtering targets for use in thepresent invention may be made of any sintered substances to be producedby mixing a source material of a II-IIIb-VIb compound with singlesubstances or compounds of a luminescent center element and of the groupIIIb element constituting the source material followed by sintering theresulting mixture. It is desirable that the density of the sinteredtargets for use in the invention is at least 75%, in terms of therelative density, of the density of the single crystal of the sourcematerials. Using the targets that meet the conditions, the unevenness ofthe luminescent center element in the luminescent layers to be formed bysputtering is reduced and the concentration of the luminescent centerelement in the formed luminescent layers is stabilized high, whereby thefluctuation in the emission luminance of the formed luminescent layersis prevented.

In case sintered targets of MGa₂ S₄ :Ce with excess Ga₂ S₃ are used toform luminescent layers of MGa₂ S₄ :Ce by sputtering, for example, as inU.S. Pat. 5,309,070, the luminescent layers formed may have elevatedluminance but the luminance varies in different luminescent layers dueto the fluctuation in the concentration of the luminescent centerelement in the luminescent layers. The techniques invented in the secondand third embodiments are effective in solving such problems in theprior art.

While the present invention has been shown and described with referenceto the foregoing preferred embodiments, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:
 1. An electroluminescent device comprising:a pair ofelectrodes; and a luminescent layer disposed between said electrodes,said luminescent layer comprising CaGa₂ S₄ with Ce incorporated as aluminescent center and having a film quality which presents a ratio ofan X-ray diffraction peak intensity for a (200) reflection of CaS to anX-ray diffraction peak intensity for a (400) reflection of CaGa₂ S₄ thatis 0.1 or less.
 2. An electroluminescent device according to claim 1,wherein said electroluminescent device comprises a substrate, a firstelectrode making up one of said electrodes and located on saidsubstrate, a first insulating layer located on said first electrode,said luminescent layer located on said first insulating layer, a secondinsulating layer located on said luminescent layer and a secondelectrode making up another of said electrodes and located on saidsecond insulating layer, a side of said electroluminescent devicethrough which an emitted light from said luminescent layer is emittedbeing optically transparent.
 3. An electroluminescent devicecomprising:a pair of electrodes; and a luminescent layer disposedbetween said electrodes, said luminescent layer comprising Ca_(1-p)Sr_(p) Ga₂ S₄ with Ce incorporated as a luminescent center and having afilm quality which presents a ratio of an X-ray diffraction peakintensity for a (200) reflection of CaS to an X-ray diffraction peakintensity for a (422) reflection of SrGa₂ S₄ that is 0.1 or less.
 4. Anelectroluminescent device according to claim 3, wherein a value p in thechemical formula Ca_(1-p) Sr_(p) Ga₂ S₄ is from 0.15 to 0.6 inclusive.5. An electroluminescent device according to claim 3, wherein saidelectroluminescent device comprises a substrate, a first electrodemaking up one of said electrodes and located on said substrate, a firstinsulating layer located on said first electrode, said luminescent layerlocated on said first insulating layer, a second insulating layerlocated on said luminescent layer and a second electrode making upanother of said electrodes and located on said second insulating layer,a side of said electroluminescent device through which an emitted lightfrom said luminescent layer is emitted being optically transparent.